Protective device in amplifier circuit



Nov. 26, 1935. J. F. cooK, JR

PROTECTIVE DEVICE IN AMPLIFIER CIRCUIT Filed July 7, 19:51

INVENTOR JOSEPH F. COOK Jr.

BY 'Arro NEY Patented Nov. 26, 1935 UNITED STATES PATENT OFFICEPROTECTIVE DEVICE IN AMPLIFIER CIRCUIT Application July 7, 1931, SerialNo. 549,124

3 Claims.

This invention relates to an improved amplifier circuit, and moreparticularly to improvements in power amplifier circuits.

The object of the invention is to provide a proper means of protecting apower amplifier circuit from building up high frequency, high voltagesurges which build up in the plate and grid circuits of amplifiers.

Another object of this invention is to provide adequate protection tothe apparatus used in an amplifier circuit such as transformer windings,local wiring, etc. The present invention is of particularly greatimportance in large power amplifier circuits, for the reason thatinterruption of service in such cases usually affects a large group ofusers.

Broadly, the invention consists of placing a neon lamp or other gasdischarge tube in an amplifier circuit in such a manner that if a surgeor oscillation is set up in the circuit to be protected,- such surgesand oscillations are suppressed thus preventing the building up ofsurges which are liable to result in damage to the transformer windings,and the associated wiring.

Local oscillations in an amplifier circuit cause undesirable resultswhich is known as motorboating,-a low frequency oscillation with veryhigh frequency components due to the wave form of the voltage induced.When using large power tubes in either a one-sided transformer, or in apush-pull circuit, the extent that oscillations may be set up is suchthat transformer windings and other associated parts of the wiring maybreak down, because when such voltages are set up in the grid circuitpower is supplied which causes insulation puncture to be followed up bypower arcs.

The so-called motor-boating mentioned above is probably due to gridemission, and while the phenomenon is not generally understood apossible explanation thereof is that a charge is built up around thegrid of the tube which causes it to block, or, in effect, the chargeincreases the negative potential on the grid. When this occurs, theplate current cannot flow. If this condition prevails for any length oftime, the space charge is reduced at a very great rate, causing a rapidbuilding up of plate current with the resultant induced high voltage inthe plate windings. The periodic repetition of this action causesmotor-boating and the setting up of oscillations.

The magnitude of the charge around the grid varies directly with thepressure of internal gases existing for any reason. The frequency ofoscillations or motor-boating is probably controlled gas pressure,internal interelectrical capacities and external circuit constants.

The present invention is adapted to eliminate v ltages resulting fromany cause, which would 2 greater than the negative bias on the grid oftube. As discussed above surges and oscilla- 5 cause the grid to gopositive, hence, to prode proper protection, in such cases, the invennprovides a neon tube having a normal b: akdown voltage not greater than110 percent the grid bias at which the tube is working connected in thecircuit so that the grid voltage is not impressed on the gas dischargetube. It is to be understood that the protective device may be placed inthe plate circuit, however, a more effective protection of the circuitis obtained when the neon tube is placed in the grid circuit, than whenplaced in the plate circuit, although the higher the plate circuitimpedance, the more effective the protective gas discharge tube becomes.

Other objects and features of the invention will be apparent from thefollowing detailed specification when read in connection with the drawinwhich,

Figure 1 is a portion of an amplifier circuit in which in accordancewith the invention protective lamps are provided;

Figure 2 is a. plan view of a convenient mounting of a pair ofprotective lamps; and,

Figure 3 is an end View of the mounting shown in Figure 2.

In Figure 1, a pair of space discharge tubes 5, 6 are shown connected inpush-pull. Each of tubes 6, 6 includes a grid element 7, an anode 8 anda cathode 9. The output circuit of the pushpull arrangement includes anoutput transformer II, the secondary of which is provided with taps l2,l3 and M for obtaining various impedance values. The input circuit ofthe push-pull arrangement includes an input transformer 5 across thesecondary of which there is connected in accordance with the presentinvention a pair of protective lamps or neon tubes 3, 3 in series, thecommon point of which is grounded at H). The various anode, cathode andbiasing sources have been purposely omitted in the drawing in order tomore clearly present the invention.

Referring to Figures 2 and 3, two protective lamps or neon tubes 3, 3are shown mounted in standard sockets 2, 2. The sockets are of wellknown construction and are adapted to be mounted on the upright portionof an L frame I which may be suitably held in position by screws orrivets adapted to be passed through holes l5, in the base plate of frameI. Suitable leads 4, 4 are provided for the various connections to thetube sockets.

It will be seen that in the arrangement shown in Figure 1 should surgesor oscillations be set up in the circuit, they will be suppressed by theprotective tubes 3, 3 inserted across the input of the amplifier tubesbefore any damage results to the amplifier circuit.

It has been found that with an output tube capable of delivering anundistorted output of approximately 4000 to 5000 milliwatts and avoltage amplification of 3.8, when the plate current is about 55milliamperes, and 450 volts are impressed on the plate and 84 volts gridbias, a neon lamp which becomes effective at about 90 volts is the besttype of gas discharge tube for proper protection.

While the invention has been shown in connection with a push-pullamplifier circuit it is to be clearly understood that the invention islicable also to amplifier circuits comprising single tubes in which casethe protective tube is connected across the input of the tube.

I claim:

1. In an amplifier system, a space discharge device having an inputcircuit and an output circuit, grid biasing means in said input circuit,circuit protective means comprising gas discharge means connected acrosssaid input circ it, said means being adaped to suppress volt in saidsystem greater than the n the input of the space discharge device.

2. In an amplifier system a pair of space discharge devices each thereofbeing provided with an anode, a cathode and at least one grid electrode,push-pull input and output circuits therefor, means for biasing the gridelectrode of each of said devices to thereby control their operatingcharacteristics, circuit protective means comprising a gas dischargepath connected eifectively across the input circuit, said path beingadapted to discharge at a slightly greater potential than the biaspotential applied to said devices whereby said system is protected fromhigh voltage surges.

3. In a high power amplifier system which ineludes at least one spacedischarge device provided with an input circuit and an output circuitand wherein the said space discharge device is biased at a predeterminedpotential so as to control the characteristics thereof, means forprotecting the amplifier and associated circuits and elements from theeffects of abnormally high voltage surges and the like which comprise anormally non-conductive electrical path shunted across the inputcircuit, said path having a pre determined breakdown voltage and beingarranged so as to by-pass damaging high voltage surges when the voltagesurges exceed a predetermined value to thereby prevent such voltagesurges from passing through the portions of the amplifying system liableto be damaged, the normally non-conductive electrical path being adaptedto breakdown at a voltage which is slightly greater than the biasingvoltage of the discharge device.

JOSEPH F. COOK, JR.

